Through their daily feeding migrations, migratory waterfowl can transport significant quantities of nutrients from agricultural fields to aquatic ecosystems where they roost. This nutrient subsidy is a management concern because it can potentially alter ecosystem productivity and affect water quality. We examined the consequences of nutrient loading by snow geese in a shallow reservoir in southeastern Pennsylvania. Each spring, over 100,000 snow geese spend 2-6 weeks at this reservoir prior to migrating north. Samples collected immediately upstream and downstream of the reservoir suggested that geese substantially increased total phosphorus concentrations, as expected due to the high phosphorus content of guano. In contrast, total nitrogen concentrations downstream were lower than upstream. Bi-weekly nutrient bioassays conducted on water collected upstream and downstream indicated that primary productivity downstream of the reservoir was limited by nitrogen, whereas water flowing in to the reservoir was limited more by phosphorus. This pattern of nutrient limitation persisted throughout the summer, even after geese had left. Therefore, nutrient inputs by snow geese appeared to cause long-term changes in the relative importance of nitrogen and phosphorus as limiting nutrients.